This invention relates to improvements in the performance of integrated circuit amplifiers. More particularly, this invention relates to improvements in the input stage of an integrated circuit amplifier which allow the amplifier's bandwidth and stability to be improved without adversely affecting its settling time.
The high-frequency performance of conventional integrated circuit operational amplifiers is typically limited by various phase delays introduced into the signal path by elements of the circuit. For example, one of the most important limiting factors results from the poor frequency characteristics of lateral pnp transistors in the signal path. In known conventional multi-stage integrated operational amplifiers, such as the LM118 commercially available from National Semiconductor Corporation and other manufacturers, lateral pnp transistors are used in a level shifting and differential-to-single-ended conversion stage which is placed between the differential input stage and the main gain stage. The poor frequency characteristics of the lateral pnp transistors introduce a pole into the open-loop transfer function of the operational amplifier. This pole severely limits the unity-gain frequency of the amplifier and introduces excess phase shift which degrades the stability of the amplifier.
A popular method of overcoming (at least in part) the limitations imposed by the pnp transistors has been the use of feedforward compensation. The intention in this type of compensation is to bypass the level shifting stage at high frequencies using a feedforward capacitor. The level shifter lends itself well to this procedure because its output is a high impedance node. Such a feedforward capacitor is used, for example, in the previously referenced LM118 circuits, in which the feedforward capacitor couples a high frequency signal from the output of the non-inverting input side of the differential input stage directly to the single-ended output of the level shifter.
In addition, conventional feedforward amplifiers such as the LM118 eliminate one-half of the high frequency signal through the slow lateral PNP's. The amplifier is "single-ended" with a shunt capacitor coupled to the output of the inverting input side of the differential input stage. This, in conjunction with feedforward compensation, has been used to greatly extend the useful bandwidth of conventional operational amplifiers.
In the conventional circuits that use the above-described shunt capacitor, the shunted output of the differential input stage is provided to the level shifting stage as a reference voltage source to maintain a bias balance in the differential level shifter. As is well-known, it is important that the reference voltage provided to one differential input of the level shifter stage should track the "non-signal dc component" of the other differential input of the level shifter stage in order to keep the amplifier's input offset voltage as small as possible. As used herein, the phrase "non-signal dc component" generally refers to the component of a voltage or current in an amplifier circuit that would appear if a set of identical common-mode inputs were applied to the amplifier (i.e., as if the differential input terminals of the amplifier circuit were tied together), and those inputs were within the common-mode operating range of the amplifier. When differential input signals are applied, the current or voltage may further include a "signal component."
Disadvantageously, the shunt capacitor has its own adverse effect on the performance of the operational amplifier. The shunt capacitor causes a doublet to be formed in the open loop response of the amplifier. The pole and zero in this doublet are separated by an octave and cause the amplifier to have poor settling time characteristics; i.e., there is an increase in the time taken for the amplifier output to settle to within a certain tolerance range of its final value after a step change in the input.
Applicants have recognized that the performance of a conventional operational amplifier using feedforward compensation can be improved by providing a novel reference voltage generator in the input stage of the amplifier. The reference voltage tracks the non-signal dc component of the output voltage of the differential input stage, but has no differential signal present and does not require a shunt capacitor. This eliminates the doublet from the amplifier's transfer function caused by the shunt capacitor. The improvement provides wider bandwidth without the settling time problem. Applicants have further recognized that the performance of other types of integrated amplifiers, such as an instrumentation amplifier, can be improved in a similar manner.
Another well-known limitation to the high-frequency capability of a conventional integrated circuit amplifier is the phase shift introduced by the current mirror circuit used to perform differential-to-single-ended signal conversion. The current mirror circuit introduces a pole into the overall amplifier response that erodes the amplifier stability for a given amplifier bandwidth. The effects of the delay through the current mirror have not been reduced in conventional integrated circuit operational amplifiers.
In a co-pending application Ser. No. 07/673,475 entitled "An Improved Amplifier Level Shift With Gain Enhancement" and filed concurrently herewith, applicants provide a novel non-differential level shifting stage which can be used to enhance the gain and input range of an amplifier incorporating the present invention and which addresses the above-mentioned problem of delay in the current mirror circuit. The novel non-differential level shifting stage takes advantage of the single-ended signal provided by the present invention, allowing such a signal to be level shifted without having to pass through a current mirror.
In view of the foregoing, it is an object of the present invention to provide a reference voltage generator which tracks the non-signal dc component of the output of a differential input stage and provides a reference voltage to a level-shifting stage so that feedforward compensation can be used to provide extended bandwidth without settling time problems.